Deep-hole electrode



1955 J. s. LARKlNS, JR 2,715,172

DEEP-HOLE ELECTRODE Filed NOV. 28, 1952 I N VEN TOR. \7 %/1 .5.,(driz'ms, \72

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United States Patent DEEP-HOLE ELECTRODE John S. Larkins, In, Detroit,Mich., assignor to Elox Corporation of Michigan, Clawson, MiclL, acorporation of Michigan Application November 28, 1952, Serial No.323,101

6 Claims. (Cl. 219-) This invention relates to electrodes for use withelectrical-discharge machining apparatus and particularly to electrodesfor drilling relatively deep holes. Drilling of holes in hard alloys byelectrical-discharge machining, sometimes referred to as arc machiningor spark machining, has become increasingly important in recent years.Examples of the type of apparatus used are illustrated and described inMcKechnie Patent No. 2,501,954 and in Harding Patent No. 2,592,894.While this apparatus was originally developed primarily for use inremoving broken drills, taps, reamers, etc., from castings in which thebroken pieces had become embedded, it was recognized that the methodused in removing the broken tools, by disintegration of the tool itselfwithout damage to the casting, could also be applied to the drilling ofholes in metal alloys too hard for machining by conventional methods andapparatus.

Applications of the method to the machining of hard alloys have beensuccessful, but it has been found that accuracy of dimension decreasesrapidly as the depth of the drilled hole increases. This factor hasseriously limited the usefulness of the method, particularly in itsapplication to the drilling and tapping of deep holes in tungsten alloysand other extremely hard metals.

In electrical-discharge machining, the electrode is supported at oneend, usually but not always in vertical position, and is vibratedaxially at high frequency in such relation to the work that electricaldischarges occur between the electrode and the work causing smallparticles of the workpiece to be eroded away. The preferred way ofremoving the eroded particles and keeping the workpiece area cool is toforce a stream of coolant fluid through the electrode, which is tubularin construction. The coolant flows outwardly of the hole around theelectrode thereby cooling the area of the workpiece adjacent theelectrode and carrying away the eroded particles. Speed of drilling isimportant and requires that the coolant flow be of considerable volumeand unimpeded. It is therefore obvious that there must be some clearancearound the electrode to permit outward flow of the coolant, and accuracyrequires that the clearance be as small as possible.

It was recognized that, in drilling deep holes, the electrode could becoated with a dielectric material which material would contact the boreof the hole during the operation. The dielectric material preventselectrical discharge between the body of the electrode and the wall ofthe hole, and the electrode is guided and restrained from radialvibration. Such a coated electrode, however, fills the hole entirely andleaves no clearance for outward flow of coolant. In addition, thecoating wears off rapidly in places and permits discharge between thebody of the electrode and the wall of the hole with consequent erosionof the bore.

My invention has for its principal object the provision of an electrodefor drilling deep holes by electricalclischarge machining, whichelectrode is guided against radial vibration and yet does not impedecoolant flow.

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Other objects and advantages will become apparent from a reading of thefollowing specification, which taken in connection with the accompanyingdrawings, discloses preferred embodiments of the invention.

In the drawings,

Fig. 1 is a fragmentary elevation of an electrode embodying theinvention;

Fig. 2 is a sectional view on an enlarged scale taken on line 2-2 ofFig. 1;

Figs. 3 and 4 are similar views of a modified form of the invention;

Fig. 5 is a fragmentary elevation of a second modified form of theinvention;

Fig. 6 is a sectional view on an enlarged scale taken on line 66 of Fig.5 and Fig. 7 is a sectional view on a similar scale taken on line 7-7 ofFig. 5.

Referring now to Figs. 1 and 2, it will be seen that the electrode 10comprises a hollow cylindrical member having an axial passage 11surrounded by a wall 12. The electrode may be of any suitableelectrically conductive material, but copper or copper alloy ispreferred. A guide element 13 of dielectric material is wrapped aroundthe electrode and fixed against displacement. The guide element in theexample shown consists of 25 lb. test nylon fishing line and is wrappedhelically around the electrode cylinder with a 42 inch pitch.

A thin coating of dielectric material 14 is placed over the entirestructure as shown in the sectional view. This coating may be ofvarnish, resin, or the like, and is of a thickness on the order of.OO2-.005 inch. The coating secures the guide element in place andretains it against displacement during axial vibration of the electrodein drilling.

While nylon fishline is readily available and convenient to use as aguide element, other materials such as glass, silicon, impregnatedcotton, etc., can be satisfactorily employed. Instead of wrapping anelement about the electrode, the guide element may be sprayed or brushedon in the form of a helix or other form, it being only necessary toprovide a continuous passage for flow of coolant.

The electrode body is unwrapped at one end (not shown) for a distance ofapproximately 1% inches to provide for chucking.

In use, the apparatus is adjusted to drill a hole sufficiently largerthan the body of the electrode such that clearance for the guide 13 willbe provided. As the electrode enters the hole, the guide engages thewall thereof and maintains the electrode centered in the hole during itsworking vibration stroke. The electrode may be withdrawn for inspectionpurposes and re-inserted in the hole with perfect centering. Duringoperation of the drill, the guide permits coolant to flow withoutobstruction and prevents contact of the body of the electrode with thewall and prevents radial vibration of the electrode.

With the electrode just described, it is possible to cut holes ofconsiderable depth with almost perfect dimensional control and almostperfect control of roundness.

Figs. 3 and 4 illustrate a modification wherein the guide element 15 ismolded in a helical groove 16 formed in the electrode body. In thisform, the guide is partially recessed and does not require as muchclearance in the hole being drilled. The element 15 may be molded frompowdered or liquid material, preferably by use of heat. The guideelement 15, in this form, being molded in the groove 16, does notrequire an adhesive coating to retain it in place, but such a coating 14is preferably used to prevent erosion of the hole by discharge from thebody of the electrode, particularly when the coolant solution used is oflow dielectric strength.

Thisform of the invention permits the drilling of deep holes withextreme accuracy, the clearance beingless'than that necessary with themodification just described because of the recessed guide member.

A third modification is illustrated in Figs. 5, 6 and 7. In this form,the guide means comprises a plurality of concentric elements '17. Theseelements are of annular form and are preferably but not necessarilyrecessed in concentric grooves 18 cut in the cylindrical body of theelement. The elements 17 may be preformed of resilient material andstretched to mount them in the grooves or they may be molded in placefrom powdered or liquid dielectric material. An insulating coating 14may be used or not as desired.

-To permit passage of coolant, the rings 17 have 'slots 19 formedtherein. Any number of slots may be'used, such that the coolant flow isnot unduly restricted.

With electrodes of the type above described used with properly designedapparatus,'it is possible to drill deep holes in hard alloys withextremelyclose tolerances'with respect to diameter and roundness Holesof irregular shape may be cut with electrodes of propershape.

I claim:

1. An electrode for cutting deep holes in metal workpieces by electricaldischarge therefrom comprising, an elongated'body member of electricallyconductive material, a passageway in said body member to provide flow ofcoolant into'the hole, a guide member of dielectric material secured'tosaid-body member and adapted to engage'the wall of said hole thereby toprovide clearance between said wall and said body for flow of coolantout cutting.

2. The combination set forth in claim 1 wherein the guide memberconsists of an element disposed helically around said body member.

3. The combination of claim 1 wherein the body member is provided with ahelical groove in which the guide member-is partially recessed.

4. The combination of claim 1 wherein the guide member consists of aringdisposedconcentrically around s'aidbodymember.

5. The combination of claim 4 wherein the body member 'is provided witha groove in which said ring is partially recessed.

6. An electrode for cutting deep holes comprising a body member ofelectrically conductive material having an internal coolant passageway,a guide member of dielectric material secured to said body member insuch mannerthat it'willengagethe wall of thelhole'thereby spacing thebody member'therefrom, said guide member providing apassageway for flowof coolant between said body-member and said wall, and a coating ofdielectric material surrounding said body member.

Rferen'ces Cited in the file of this patent IJNITED STATES PATENTS 1,341,647 Jones June 1, 1920 1" ;944',75'3 Mathias Jan. 23, 1934 22 ,013Smith Jan. 13, 1942 2,494,029 Bertalow et' a1. Jan. 10, 1950

